The advent of optical fibers for communication purposes was first thought to provide ready means for transmitting secret information without elaborate security precautions. Access to the information was considered impossible since the fiber would have to be broken in the process. Several ingenious methods for gaining access to the fiber core without breaking the fiber have evolved since the first early optical communication systems were devised.
One method for gaining access to information transmitted within the fiber core consists of etching away a portion of the optical cladding material and coupling into the light path by submersing the fiber in a material of high refractive index. By this method light would readily transmit from the fiber core through the high index of refraction material where it could be optically received and monitored. Since this method of tapping light energy could be detected at the optical receiver end of the communications system, various methods were required for encoding the information data in order to preserve the system's security.
The use of complex encoding equipment and circuitry at the optical transmitter end and the use of similar decoding apparatus at the optical receiver end adds to the large quantity of communications equipment employed within the optical communication system. The purpose of this invention, therefore, is to disclose methods and apparatus for providing a simple and effective secured optical communications system.